When Jack Frost calls

The spring of 2002 was a cold one, with Jack Frost making many an unwanted visit to the country’s fruit and vegetable growers.On Friday October 4, 2002, an intense low east of New Zealand moved away and a weak ridge of high pressure spread over the country. Strong southerly winds on the western flank of the low had blown a deep layer of cold air over New Zealand. This, combined with the light winds and clear skies in the ridge, cre­ated the perfect conditions for a hard frost.

Soon after midnight, dozens of helicopters were in the air from Marlbor­ough to Bay of Plenty in a bid to protect vineyards and orchards from harm. How can a helicopter prevent frost damage? On a clear night, the Earth’s surface radiates heat out to space, and so becomes colder.

The ground then cools the air that is in contact with it. If there is no wind to mix the air, a shallow cold layer builds upwards from the ground, capped by relatively warm air above it. If the air is cooled be‑ low 0° C then some of the water vapour in the air is deposited as ice crystals on the surface of things,forming frost.

If water inside plants freezes, it expands, damaging the plants cells. On nights when frost threatens, helicopters are used to create a downdraft that mixes warm air into the cold surface layer. The mix‑ing may raise the temperature only a fraction of a degree above zero for 20 minutes or so before it has to be repeated. At over $1000 an hour per helicopter, it is an expensive exercise, but well worth it if the crop can be saved.

Unfortunately, on October 5 the layer of cold air was too too deep in some places, and millions of dollars of damage occurred,despite the helicopters’ best efforts. Particularly badly hit were the early-floweringgold kiwifruit in western Bay of Plenty. One grower said that he expected to lose $250,000 worth of fruit off just one 2 ha block. Also badly hit were chardon­nay vineyards in Hawke’s Bay and Wairarapa. A grower in Hawke’s Bay lost 16,000 basil plants, even though the plants were growing inside plas­tic tubes. He also lost 6000 courgette plants.

Other measures can be taken against frost. These include deploying giant fans, which work on the same principle as helicopters but rely on the cold layer not being too deep, and lighting smoke pots, which warm the air and pro­duce smoke to slow down the radiative cooling.

Perhaps the most surpris­ing protective measure is the use of sprinklers to cover plants in a layer of water. When water freezes it releases latent heat, which helps to keep the tempera­ture inside the plant just above zero, so the plant is protected even though encased in ice.Cold springs are the norm in El Niño years. Oc­tober 2002 had an average temperature of 11.0° C,

As well as suffering repeated frosts, some or­chards and vineyards were also badly affected by hail. Combined with the loss of lambs killed by late snow in Otago and Southland, plus the effects of slow grass growth, the total losses to farmers were estimated to be around $400 million. For many growers, the only benefit from the cold was the reduced numbers of pests.which was 1.1° C below nor­mal, making it the coldest October since the record-breaking El Niño of 1982. Some growers, accustomed to the warmth of the past two decades, were caught off guard by the sudden cold snap. Figures released by HortResearch showed that in Marlborough between 1930 and 1980 there were an average of 86 frosts per year, but from 1986 to 1999 there were only 56 frosts a year.

Besides hiring helicop­ters, switching on fans and lighting fires, there is another tactic which can be used to beat Jack Frost: plant in places where he doesn’t visit. The provision of data for this sort of agricultural planning was one of the prime reasons for setting up recording stations to monitor weather con­ditions in the 1800s.

Temperature ob­servations from these stations have been used to make maps of frost parameters, such as the aver­age number of days between the last frost in spring and the first frost in the following autumn. Although these maps show only broad-scale patterns, one

feature is clear: the further inland, the greater the risk of frost. In fact, differences as large as three months oc­cur between the dates of first frosts at coastal stations and places 100 kilometres inland, whereas the difference be­tween the very north and the very south of the country—a distance of approximately 1400 km—is only about one month.

There are two reasons why this should be so. First, on a clear winter’s night the sea surface is much warmer than that of the land. So, if a breeze is blowing from sea to land, it prevents frost form­ing in coastal areas. Second, places further inland tend to be higher above sea level and therefore have colder temperatures, because tem­perature generally decreases with altitude.

The trouble with assess­ing frost risk using such broad parameters is that frost can vary dramatically from one paddock to the next, and it is not feasible to erect thermometer screens at hundred-metre intervals across the landscape.

This is where satellites, the meteor­ologist’s wonder toy of the late 20th century, can help. High-resolution ra­diometers carried on the latest National Oceanic and Atmospheric Administration (NOAA) polar-orbiting sat­ellites can measure the heat radiated from clouds or from the Earth’s surface to a reso­lution of 1 km. Using radi­ometer measurements taken over Otago on a number of cloud-free nights, some in­genious maths and a power­ful computer, two scientists at the National Institute for Water and Atmospheric Research, Andrew Tait and Xiaogu Zheng, were able to interpolate between the sparse climate stations and produce frost-risk maps with a resolution of 1 km.

Going one better, they then got the computer to digest a detailed topographic map, and, allowing for alti­tude, aspect, and ponding effects, were able to interpo­late down to a resolution of only 50 metres.

Among the interesting detail that has emerged from their work is the extent to which the sloping sides of some valleys have fewer frosts than the valley floors, where cold air tends to col­lect. If, however, the valley has a lake in it, the moderat­ing effect of the relatively warm water can clearly be seen (see map below).

This study, which was funded by the Otago Regional Council and the Foundation for Research, Science and Technology, will be invaluable for growers planning new agricultural projects in Otago, or wishing to expand existing ones.

As we enjoy the fruits of the latest technology, it is interesting to glance back up the great glacier of time and unravel an old mystery: how did Jack Frost get his name? In Scandinavian folklore, frost is said to be brought by the son of the god of the winds. He has two names: Jokul, meaning icicle, and Frosti, meaning frost. Over time, these names have been combined in English to become Jack Frost.